In photovoltaic devices, electrical energy is converted from light via the photoelectric effect and resulting charges are collected via pn junctions in semiconductor substrates for current generation. PN junctions can be formed by means of diffusing dopants into the bulk semiconductor material. The diffusion process can occur from a dopant vapor ambient, for example, phosphine or POCl3, or from solid source releasing dopant vapor. Another process can employ a dopant layer coated on the substrate, which, upon heating or firing, causes the dopant to diffuse from the dopant layer into the substrate. Another important process in solar cell fabrication is thin film deposition, such as the coating of passivation or absorber layer.
In-line fabrication processes, such as in-line doping diffusion or in-line thin film deposition, are preferred processes for minimizing cost and toxicity. For example, in a typical in-line diffusion process, the substrate is coated with a dopant containing layer and subsequently, the dopants from the dopant layer are diffused into the substrate in a furnace. The coating of the dopant source can be accomplished by spraying, dipping, spin-on, or condensation of a dopant-containing chemical, in liquid or gaseous form, with or without solvent, and with or without carrier gas. Such systems can be difficult to control with respect to uniformity and doping levels, particularly on textured surfaces. Also these processes often require significant excess chemicals, driving up the cost of production.